1. Field of the Invention
The present invention provides a thin semiconductor film transferring method; more particularly, it is a layer transferring method for transferring a thin film onto a dissimilar material, a thin film of the same area as the wafer, of a sub-micron thickness, and of the thickness and flatness at the VLSI standard, and with low defect in the film density.
2. Description of the Prior Art
The wafer bonding technology demonstrates to join two silicon wafers by bonding the silicon atoms on the surfaces of the wafers without glue was publicized by J. B. Lasky at the IEDM meeting held by IEEE in 1985. The said technology can join two single crystal wafers of very different lattice parameters immaculately without any glue, and the resulting bonding strength of the bonding interface can be as strong as the substrate. For this reason, the technology satisfies the strict standards for the uncontaminated bonding interface for modem electric materials and opto-electronics materials. Because of the advantages described above, the wafer bonding technology has drawn much attention after its disclosure.
Later W. Maszara utilizing the principle of elastic mechanics introduced the insertion method to measure the bonding strength in 1988. Insertion method was a simple and quick method to check the bonding qualities of two bonded wafers.
Maszara also used a heavy doping P+ type silicon layer as an etch-stop layer to fabricate submicron thickness silicon on insulator such as silicon dioxide to form the silicon on insulator (SOI) structure material. That extended the applicable field of the wafer bonding technology to advanced electric materials, opto-electronics materials, and microelectro-mechanical systems (MEMS). However, the wafer bonding technology still has some disadvantages, such as the existence of the etch-stop layer on the surface of the wafer after etching and the uniformity of etching on different sites of the etch-stop layer caused the problem related to the total thickness variation (TTV). In addition, the process requires too much time-not only many supply substrates are wasted, and some environmental pollution problems may occur due to the discarded fluid. At that time, the Separation by Implantation Oxygen (SIMOX) method was also energetically developed in making SOI material. Because the thin film formed by SIMOX method has a perfect uniform thickness, wafer bonding technology may lost its leadership in the field of SOI wafer production if its TTV value can not be improved.
Until the end of 1994, M. Bruel announced the success of the development of a new thin film transfer technology so called Smart-Cut process. With the approach of the Smart-Cut process, the thickness of SOI material can be made as well as with the SIMOX method. According to the claims of U.S. Pat. No. 5,374,564, an implantation process is performed first to implant high dosage of hydrogen ions or gas ions from VIII group into a supply substrate, and the supply substrate is bonded onto a demand substrate. A thermal treating process is then performed to gather these gaseous ions in the implantation layer, and causes the formation of micro-bubbles. The temperature is continually increased to increase the pressure of bubbles formed by incorporating micro-bubbles so as to separate the implanted thin film from the supply substrate and transfer onto the demand substrate, and the thin film on the demand substrate is formed. Because of the advantages of the Smart-Cut process, such as uniform thickness of thin film, less defects in density, no wasted materials, the unharmful released hydrogen, and that the supply substrates can be reused, the thin film transferring technology has been developed quickly with the wafer bonding method.
But the Smart-Cut process has some disadvantages, such as the thermal stresses generated from thermal treating process or the low manufacturing efficiency due to the time to reach sufficient strong bonding strength at lower temperature prior to layer splitting. The thermal treatment of the Smart-Cut process is performed by a heat source to activate the implanted hydrogen ions to be gathered to form bubbles, so that the ion separation layer is separated due to the expansion of the bubbles, and then the thin film transferring is achieved. When the thermal treatment is performed, the heat is first transmitted to the surface of the bonded substrates to rise the temperature of the surfaces, and then the heat is transmitted to the inner of the substrate due to the temperature difference between the surface and inner of wafer. That results in the following five disadvantages:
1. According to the low temperature bonding technology, the initial bonded wafer pairs need a long annealing time to enhance the bonding strength. That is, before the bonding strength is strong enough to gather bubbles to form a force to separate the wafers, the temperature must be controlled lower than 450xc2x0 C. which is the temperature that the hydrogen ions cause distinct bubbles. Because the initial bonded wafer pairs must be controlled lower than 450xc2x0 C. to perform an annealing process, the annealing time is too long, and the throughput is decreased.
2. The substrates must be heated a whole to make the uniform rising of the temperature of the substrates. And in order to achieve the expected result, the heating temperature in the prior art is about greater than 500xc2x0 C. However, when the substrates are dissimilar materials, a large thermal stresses will occur to destroy the bonding structure due to the different thermal expansion coefficients of dissimilar materials, furthermore, cracks may occur in the bonding structure before layer transferring to destroy the bonded materials.
3. The instant heat amount is transmitted unequally, so the instant temperature of each point within the substrates is unequal. That results in the timing and the position of thin film separation varies, and inner stresses cause granulation of the transferring interface, even more, cause many interstices.
4. The thermal efficiency of the annealing process which transfers thermal energy into kinetic energy is quite low, much energy of the heat source is wasted, so the manufacturing cost is increased due to the maintain in high temperature.
5. For some materials, such as Al2O or LaAlO3, the Smart-Cut process is not able to form enough micro-bubbles to separate the thin films.
The present invention directly excites the implanted ions or molecular ions by the high frequency alternating electric or electromagnetic field to increase the collision frequency. Thus, the micro-bubbles generate and expand quickly so as to transfer the thin film from the supply substrate to the demand substrate. It has been proved that the present invention can reduce the cost, increases the throughput, and improves the quality of products.
It is an object of the present invention to a thin film transferring method for a thin film on a substrate in a similar or heterogeneous material structure, having a transferring area equal to the size of the wafer, having a thickness of sub-micron degree, having a total thickness of VLSI degree, and with low defect density.
In the preferred embodiment of the present invention, an ion implantation process is performed to implant ions or molecular ions into the supply substrate and to form an ion separation layer in the supply substrate. A wafer bonding process is then performed to bond a demand substrate onto the supply substrate to form a bonded structure. Then the bonded structure is placed into a high frequency alternating electric field, or a high frequency alternating electromagnetic field to perform an ions activation process. Using microwaves, radio frequency, or inductive coupled field which increase the kinetic energy of the implanted ions, molecular ions, or reactants produced by reactions between the ions and the substrate in the bonded structure the thin film transferring from the surface along the ion implanted plane of the supply substrate to the surface of the demand substrate.
For layer transfer of dielectric loss supply substrate, the present invention performs with phase-in ion implantation. The present invention first implants the ions at high temperature to form crystal fissures, and then implants ions at lower temperature into the crystal fissures to prevent the loss of diffusion of the implanted ions, and the dosage of ions is enough to form micro-bubbles to form a separation layer. Next, perform ion activation by high frequency alternating electric or electromagnetic field to form inductive energy to increase the kinetic energy of these implanted ions to gather in gas molecules to form cracks, and separation of thin film is finished. By this way, the total dosage of implanted ions can be decreased, the cost is retrenched, and the density of the defect of the thin film is improved.
Besides, the present invention is further applied in a thin film cutting process. First, perform an ion implantation to form one or more ion separation layer within the thin film. Then, perform an irradiation of the high frequency alternating electric or electromagnetic on the thin film to gather the implanted ions in gas molecules, and a separation layer is formed to separate the thin film. The thin film cutting is finished.